U.S. patent number 6,131,848 [Application Number 09/160,808] was granted by the patent office on 2000-10-17 for roadable airplane drive through an automotive transaxle.
Invention is credited to Steven Collins Crow.
United States Patent |
6,131,848 |
Crow |
October 17, 2000 |
Roadable airplane drive through an automotive transaxle
Abstract
A roadable airplane consists of a single-seat three-wheeled
vehicle with wing panels hung longitudinally on the sides of the
chassis for road use and attached transversely to a spar box for
flight. A power train is used that enables the simultaneous
powering of two rear wheels and an overhead pusher propeller for
transition from road to air travel. A transaxle for a four-wheel
automotive vehicle was found to be advantageously suitable for this
purpose. Multiple gears and differential outputs drive the wheels,
providing a normal automotive performance on the road. Multiple
gears are also available to drive the propeller, thereby providing
the function of variable-pitch performance using a fixed-pitch
propeller. The vehicle includes a standard automotive gear shift
and pedal controls for clutch, brake and accelerator operation; a
novel joystick assembly is used to steer the front wheel and to
control wing flaps, an elevator and a rudder.
Inventors: |
Crow; Steven Collins (Conifer,
CO) |
Family
ID: |
26740460 |
Appl.
No.: |
09/160,808 |
Filed: |
September 25, 1998 |
Current U.S.
Class: |
244/2; 244/50;
244/51; 244/60 |
Current CPC
Class: |
B64C
37/00 (20130101) |
Current International
Class: |
B64C
37/00 (20060101); B64C 037/02 () |
Field of
Search: |
;244/2,50,51,60,62 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Chiles, J.R., "Flying Cars were a Dream that Never Got Off the
Ground", Smithsonian, vol. 19, No. 11, pp. 144-162, Feb.1989. .
Grossmann, John, "Auto Pilots", Air & Space, vol. 10, No. 5,
Jan. 1996. .
Stiles, P., Roadable Aircraft, from Wheels to Wings, Second
Edition, Custom Creativity, Inc., Melbourne, Florida, Aug. 1994
(copy not available)..
|
Primary Examiner: Eldred; J. Woodrow
Attorney, Agent or Firm: Durando; Antonio R.
Parent Case Text
CROSS REFERENCES TO RELATED APPLICATIONS
This application is based on U.S. Provisional Application No.
60/060,883, entitled "Flying Car," filed by the same inventor on
Oct. 2, 1997.
Claims
I claim:
1. A vehicle for road and air travel comprising the following
components:
a roadable chassis including traction wheels and a pair of
wings;
a motor mounted on the chassis;
a drive train coupled to an output shaft of the motor, said drive
train comprising an automotive four-wheel drive transaxle; and
a propeller mounted on the chassis such as to provide forward
thrust;
wherein the propeller is coupled to a first output from the
automotive four-wheel drive transaxle and the traction wheels are
differentially coupled to a second output from the automotive
four-wheel drive transaxle, the propeller being coupled to the
traction wheels through the transaxle in a fixed gear ratio.
2. The vehicle of claim 1, wherein said automotive four-wheel drive
transaxle comprises multiple forward gears and a reverse gear.
3. The vehicle of claim 1, wherein said propeller has a
ground-adjustable pitch which is fixed in flight.
4. The vehicle of claim 1, wherein said propeller is a pusher
propeller.
5. The vehicle of claim 1, wherein said propeller is an overhead
pusher propeller.
6. The vehicle of claim 1, wherein said propeller has a
ground-adjustable pitch which is fixed in flight.
7. The vehicle of claim 6, wherein said propeller is a pusher
propeller.
8. The vehicle of claim 7, wherein said propeller is an overhead
pusher propeller.
9. The vehicle of claim 1, wherein said traction wheels are two
rear wheels and the vehicle further comprises a front wheel.
10. The vehicle of claim 9, wherein said front wheel is steered by
a joystick.
11. The vehicle of claim 10, wherein said joystick is adapted to
also control an elevator, a rudder and wing flaps.
12. The vehicle of claim 7, wherein said traction wheels are two
rear wheels and the vehicle further comprises a front wheel.
13. The vehicle of claim 12, wherein said front wheel is steered by
a joystick.
14. The vehicle of claim 13, wherein said joystick is adapted to
also control an elevator, a rudder and wing flaps.
15. The vehicle of claim 1, wherein said wings are removably
attached to the chassis.
16. A method of flying a vehicle including a roadable chassis with
traction wheels, a motor mounted on the chassis, a propeller
mounted on the chassis such as to provide forward thrust, an
elevator and a rudder, said method comprising the following
steps:
affixing a pair of wings to the chassis in an orientation suitable
for flight;
providing a drive train coupled to an output shaft of the motor,
said drive train comprising an automotive four-wheel drive
transaxle having a first output coupled to the propeller and a
second output differentially coupled to the traction wheels, the
propeller being coupled to the traction wheels through the
automotive four-wheel drive transaxle;
operating the vehicle such as to simultaneously power the traction
wheels and the propeller coupled in a fixed gear ratio through the
transaxle; and
accelerating the vehicle to a takeoff speed and actuating the
elevator to cause the vehicle to become airborne.
17. The method of claim 16, wherein said automotive four-wheel
drive transaxle comprises multiple forward gears and the method
further comprises the step of shifting gears during takeoff to
maximize propeller and wheels thrust, and during flight in order to
alter a ratio of motor-to-propeller speeds.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
In general this invention pertains to motor vehicles and aircraft.
In particular, the invention relates to a flying vehicle with a
power train configuration that attains the driving characteristics
of a three-wheeled motorcycle and the flying attributes of a
primary class airplane.
2. Description of the Prior Art
The idea of a flying car has had great appeal ever since
automobiles and airplanes became accepted means of transportation.
The concept is attractive as an engineering challenge as well as
the fulfillment of a quest for total freedom and independence.
Several authors have commented on the concept of a flying car and
described various attempts at producing a viable machine for
personal use. See, for example, Chiles, J. R., "Flying Cars were a
Dream that Never Got Off the Ground," Smithsonian, Vol. 19, No. 11,
pp.144-162, February 1989; Stiles, P., Roadable Aircraft, from
Wheels to Wings, Second Edition, Custom Creativity, Inc.,
Melbourne, Fla., August 1994: and Grossmann, John, "Auto Pilots,"
Air & Space, Vol. 10, No. 5, January 1996.
As reported in these articles, the prior art abounds with concepts
for flying cars that look and feel like cars on the road and
somehow become airplanes for the sky. The common solution is a
modular approach to provide both automotive and aircraft
performance. Referred to as "transformers," these vehicles
incorporate alternative modules that permit the conversion of a
roadable car into a flying craft. This adaptation typically
requires the vehicle to pass through an elaborate transformation
where a sky module is added for flight.
A common objection to this type of flying cars is that they are
poor airplanes as well as poor cars. In other words, the
combination imposes excessive compromises on both the car and the
airplane aspects of the vehicle.
Other types of flying vehicles have resulted from the adaptation of
a conventional airplane to highway use, or that of a road vehicle
to airborne use, but neither have produced a satisfactory
compromise. The main problem has always been the lack of a single
power train suitable for powering both a set of wheels and a
propeller under conditions appropriate for each use. Obviously,
road use requires multiple gears, including a reverse gear, while a
propeller requires a single gear and, preferably, an adjustable
pitch for transition between take off and cruise altitudes. For a
roadable aircraft to become practically feasible, these features
must be packaged in a light unit that is also relatively
inexpensive to manufacture. The present invention discloses a novel
combination of known components that satisfies these
requirements.
SUMMARY OF THE INVENTION
One of the objectives of this invention is a self-contained vehicle
capable of being operated either as a motor vehicle on the ground
or as an airborne craft without any adaptation by the addition of
specialized modules for each function.
Another objective of the invention is a vehicle that meets current
standard regulations in the United States for motorcycle
roadability and for certification as a primary class aircraft; that
is, a roadable airplane.
Another goal is a vehicle that fits in an ordinary garage.
Another objective is a vehicle that utilizes the same motor, power
train and controls on the road and in the sky.
Still another objective of the invention is an approach that
affords flexibility of design, ease of repair, and efficient
exploitation of existing technology.
Finally, another goal is a vehicle that can be manufactured at
reasonable cost and is affordable to the general public.
Therefore, according to the foregoing objectives, the roadable
airplane of the invention consists of a single-seat three-wheeled
vehicle with wing panels hung longitudinally on the sides of the
chassis for road use and attached transversely to a spar box for
flight. The main aspect of the invention involves the use of a
power train that enables the simultaneous powering of two rear
wheels and an overhead pusher propeller for transition from road to
air travel. A transaxle for a four-wheel automotive vehicle was
found to be advantageously suitable for this purpose. Multiple
gears and differential outputs drive the wheels, providing a normal
automotive performance on the road. Multiple gears are also
available to drive the propeller, thereby providing the function of
variable-pitch performance using a fixed-pitch propeller. The
vehicle includes a standard automotive gear shift and pedal
controls for clutch, brake and accelerator operation; a novel
joystick assembly is used to steer the front wheel and to control
wing flaps, an elevator and a rudder.
Various other purposes and advantages of the invention will become
clear from its description in the specification that follows and
from the novel features particularly pointed out in the appended
claims. Therefore, to the accomplishment of the objectives
described above, this invention consists of the features
hereinafter illustrated in the drawings, fully described in the
detailed description of the preferred embodiments and particularly
pointed out in the claims. However, such drawings and description
disclose only some of the various ways in which the invention may
be practiced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic side-elevational view of the preferred
embodiment of a roadable airplane according to the present
invention.
FIG. 2 is a schematic top view of the roadable airplane of FIG.
1.
FIG. 3 is a schematic side-elevational view of the fuselage frame
of the roadable airplane of FIG. 1.
FIG. 4 is a schematic top view corresponding to FIG. 3.
FIG. 5 is a perspective view of the fuselage frame of FIGS. 3 and
4.
FIG. 6 is an enlarged partial view of the frame of the invention
showing the connection of the spar box to the frame.
FIG. 7 is an enlarged partial view of the propulsion assembly
compartment illustrating the motor, transaxle, transfer case, belt
drive, and propeller installed in the chassis.
FIG. 8 is a stand alone picture of the propulsion assembly of the
invention.
FIG. 9 is an enlarged partial view of the motor mounts in the
chassis of the invention.
FIG. 10 shows in side view the propulsion assembly of the invention
installed in the chassis.
FIG. 11 is a top view of the chassis of the invention showing two
lateral differential outputs (without the propulsion assembly) used
to power the rear wheels of the vehicle of the invention.
FIG. 12 is a side view corresponding to FIG. 11.
FIG. 13 is a partial, more detailed view of the rear-end
configuration shown in FIG. 12.
FIG. 14 is an enlarged partial view of the seat and fuel cell
support structure in the chassis of the invention.
FIGS. 15 and 16 illustrate the vehicle of the invention in side and
top views without wings.
FIGS. 17-19 show three views of the roadable vehicle of the
invention configured as an airplane.
FIG. 20 is a graph of estimated acceleration-versus-speed profiles
resulting from wheel, propeller and combined thrust.
DETAILED DESCRIPTION OF THE INVENTION
As used in this disclosure, the term "four-wheel drive transaxle"
refers to automotive transaxles with two differential outputs to
half axles and a single output to a second axle. Some four-wheel
drive transaxles provide two lateral differential outputs for the
front wheels and a straight
output connected to a separate rear differential for the rear
wheels. The heart of this invention is the realization that this
type of drive train, properly adapted with specifications suitable
for an aircraft, makes it possible to achieve the development of a
practically functional, affordable roadable airplane. In essence,
the novel idea of utilizing the same motor and transaxle for
simultaneously powering the wheels and propeller of the vehicle,
and the fact that a suitable transaxle is available from the
automotive industry, render the dream of a practical flying car
possible.
Referring to the drawings, wherein like parts are designated
throughout with like numerals and symbols, FIGS. 1 and 2 illustrate
in schematic side-elevational and top views the preferred
embodiment of a roadable airplane 10 according to the present
invention. The vehicle 10 comprises an aerodynamic body or fuselage
12 built around a three-wheeled chassis equipped with all
components necessary for flight and land operation. For road
operation, two wings 14 are attached substantially longitudinally
to the sides of the body of the vehicle 10, so as to provide
minimum wind resistance to forward motion. The wing flaps 16 (which
in fact are designed to provide the function of a flap/aileron
combination) are preferably kept folded while the wings hang from
the sides of the vehicle in order to maximize visibility from the
cockpit through the canopy's windshield 18. The vehicle has no
doors because it is low to the ground, and the driver/pilot can
easily step over the side when the canopy is raised. Since a
steering wheel would interfere with egress, steering on the ground
is accomplished with a joystick also used to control roll and pitch
in the air. Tail support booms 20 extend backward from the fuselage
frame and support a horizontal tail 22 and two vertical fins 24.
The horizontal tail 22 is low enough to lie under the fuselage wake
during flight at high angles of attack. The vertical fins 24 serve
as end plates to the tail 22, thereby increasing the effective tail
span, as well understood in the art. Rudders 26 and an elevator 28
are provided in conventional configuration.
FIGS. 3 and 4 show in side and top views the fuselage frame or
chassis 30 of the vehicle 10 of the invention. The frame 30, which
is the structural core of the vehicle, consists preferably of a
chrome-moly steel tube frame. A steel tube structure was selected
because it can accommodate various hard points intrinsic to a
roadable airplane, such as a stem tube for a front wheel and
suspension assembly, rear suspension links, spar box mounts, and a
propeller bearing tube. Steel tube frames are also easy to design
and relatively inexpensive to fabricate. In addition, chrome-moly
tubes and an unloaded composite aeroshell are believed to be the
lightest structural combination suitable to the objectives of the
invention.
To be safely operable, the frame 30 of the flying vehicle 10 must
obviously be capable of withstanding high positive and negative
loads. A planned prototype designed to withstand a positive load of
6.6 g's and a negative load of 3.3 g's is made of 4130 N steel
alloy tubes, welded together by the tungsten inert gas (TIG)
method. Round, square and rectangular tubes of several outer
dimensions and thicknesses are used. The upper longerons 32, for
example, consist of round tubes with 1.25 inch outer diameters and
0.049 inch thicknesses. The lower longerons 34 have the same outer
diameters, but are 0.035 inches thick. Most of the substantially
vertical struts 36 are 1 inch OD round tubes or 1 inch square
tubes, 0.035 inches thick. The prototype frame is illustrated in
perspective view in FIG. 5, wherein a battery 38 and a wing spar
box 40 (also seen in FIGS. 3 and 4) are shown mounted in front and
in the central section of the frame, respectively. The vehicles's
dashboard, firewall, and cabin floor are not shown. The tubular
frame shown in FIG. 5, without the battery and spar box, is
estimated to weigh about 114.8 lbs.
In order to allow for maximum flexibility and easy access to the
motor compartment 42 (see FIG. 4), the spar box 40 is attached to
the frame 30 by means of L-shaped aluminum clamps 44, as shown in
FIG. 6. Since the spar box is preferably made of aluminum, it
cannot be welded to the frame. Clamped as shown, the spar box can
be conveniently removed to gain access to the motor and transaxle,
which are mounted in the compartment 42. Aluminum saddles, not
shown in the figures, separate the spar box from the tubes.
FIG. 7 illustrates in side view the propulsion assembly of the
invention. A motor 46, a transaxle 48, a custom transfer case 50,
and a belt drive 52 form a structural unit installed in the
compartment 42 on soft mounts 54 and 56 (see also FIG. 9). The belt
drive conveys power from the transaxle to a pusher propeller. Since
it is known that such drives are prone to resonant torsional
vibrations in the absence of a flywheel and is clutch, these
components are retained in the drive train of the invention.
Utilizing a belt 60 driven by the drive 52 coupled to the
transaxle's rear output, the propulsion assembly is connected to a
propeller shaft 62, which in turn (through elastomeric couplings
shown in FIG. 8) is attached to a stub shaft inserted through
bearings in a propeller bearing tube 64. A collar is splined to the
aft end of the stub shaft, and the propeller hub is bolted to the
collar. The propeller 66 is preferably a three-blade propeller with
a diameter of 54 inches, as available from Warp Drive Inc. of
Ventura, Iowa. The blade pitch is preferably adjustable while the
vehicle is still on the ground, but it remains fixed in flight. The
preferred embodiment of the invention is based on a Stratus EA81
engine, which is a Subaru EA81 engine converted for airplane use by
Stratus, Inc., of Seattle, Wash., and a companion Gunma Manufacture
4WD4MT transaxle, as available in 1984 Subaru station wagons model
4WD-DL equipped with 1,800 cc engines. It is estimated that the
whole propulsion assembly, as shown in FIG. 8, will weigh about 404
lbs, including the bearings but not the bearing tube 64, which is
welded into the frame. The Stratus EA81 engine develops more than
100 hp at 5400 rpm and it is expected to deliver 100 hp to the
propeller 66, which is sufficient for flight of an appropriately
designed primary class aircraft. It is noted that the bell housing,
the stock flywheel and clutch of the automotive engine are retained
for the hybrid application of this invention.
FIG. 9 shows the motor mounts 56 adopted for the vehicle 10 of the
invention. The motor and transaxle rest on 45-degree rubber pads
under the bell housing, which are bolted onto steel rings 68 on the
motor mount. These pads are positioned slightly ahead of the center
of gravity of the combined motor and transaxle. The pads preferably
consist of two-piece Lord mounts, similar to those used in general
aviation. The pads are bolted through alloy C-sections, welded to a
lateral tube 70 with a 1.25 inch OD and a thickness of 0.049
inches. Square tubes 72 form a light truss on the bottom and brace
the motor against fore and aft movement. The complete assembly
weighs about 6 lbs. Tabs and Lord pads on the transfer case aft of
the transaxle (shown as 54 in FIG. 7) complete the mounting
subsystem.
Inasmuch as it consists of an automotive component, the transaxle
48 provides reverse plus four speeds forward, with an output shaft
at the rear (the four-wheel drive output in automotive
applications) that is used to drive the propeller 66. It is
expected that the roadable airplane of the invention will takeoff
in third gear, at a transaxle gear ratio of 1.193. The belted
propeller-speed reduction unit provides a further ratio of 1.389,
for a net speed reduction of 1.6571. Based on these reduction
ratios, the motor speed at takeoff is expected to be about 5000
rpm, and the propeller speed about 3000 rpm.
It is estimated that the segment 74 of longeron just above the spar
box 40 will experience an upward thrust of about 4000 lbs during a
6.6 g pull up. Thus, in order to prevent crushing, that segment is
made of 1.25 inch OD alloy tube, 0.25 inches thick. As such, the
tube segment 74 provides the necessary structural strength to the
frame 30 at an additional weight of about 4.45 lbs. It is noted
that the spar box assembly functions as a "shoulder" of the
airplane and is a focus of extreme force. Another focus of force is
the forward motor mount 56, which must withstand a download of
about 2000 lbs during a 6.6 g pull up. The motor needs to be soft
mounted and cannot be part of the support structure; so, the motor
mount needs to be sturdy. FIG. 10 shows the propulsion assembly of
the invention installed in the chassis 30. The transaxle 48
provides a rear output shaft 76 that is advantageously used to
power the propeller 66, as described above, and two lateral
differential outputs 78 used to power the rear wheels 80 of the
vehicle (the wheel axles are seen in the top view of FIG. 11, which
for clarity is shown without the propulsion assembly). Most of the
running gear components utilized for the prototype of the invention
are borrowed from existing components used in the motorcycle or
automobile industries. The front end 82, also illustrated in FIG.
12, consists essentially of off-shelf components from a racing
motorcycle except for the springs, which are selected to provide a
1.2 Hz bounce frequency considered optimal for the hybrid
application of the invention. These components include the tire,
wheel, axle, rotor, brake and caliper. While ordinary airplanes
operate on smooth runways and can utilize small tires, a roadable
airplane needs large tires to serve as filters between road
irregularities and the chassis. Accordingly, the front tire of the
vehicle 10 is a 24-in tire mounted on an 18-in motorcycle wheel,
while in the rear tires are 24-in tires mounted on 14-in automobile
wheels. The wheels and tires are chosen for the lowest possible
combined weight. It is noted that the front end 82 is estimated to
carry only about 181 lbs at maximum gross weight, considerably less
than when used in a racing motorcycle. Using such available
components, it is calculated that the complete front end 82 will
weigh about 33 lbs excluding the stem tube 83, which is welded into
the frame.
In order to be acceptable within the objectives of the invention,
the rear end of the vehicle 10 must provide conventional automotive
attributes, including power transfer to the wheels and a smooth
ride. In addition, the rear end must not protrude into the air
stream below the smooth underpan of the vehicle and must be capable
of absorbing the impact of a 10 ft/sec hard landing. Thus, both
front and rear ends are designed to bounce at a frequency of 1.2
Hz. The struts 84 in the rear (FIG. 12) are oleo-pneumatic with no
metal springs. They are of a type used for off-road vehicles and
are used as McPherson struts in the vehicle 10. As seen more
clearly in the enlarged view of FIG. 13, the struts 84 fit into
custom axle carriers 86, which are CNC-machined from 6061-T6
aluminum. Lower wishbones 88 (see also FIG. 11) are connected to
the motor bay diagonals 90 in front and the transaxle lateral in
the rear. The rear links 92 of the wishbones have gull-wing shapes
arching above the lower longerons 34 of the frame. The components
of the rear end are estimated to weigh about 112 lbs, including
wheels and tires.
FIG. 14 illustrates a seat and fuel cell mount 94, which is another
substructure that must be capable of handling high forces during
extreme maneuvers. The design includes a fuel cell 95 (seen in
FIGS. 11 and 12) lying in a compartment 96 directly under the seat
97 (see FIG. 12), so the frame around the fuel cell and under the
seat mount are one and the same. The purpose and concept of the
invention virtually require that the fuel cell be placed under the
seat because safety considerations demand that it be located ahead
of the firewall separating the seat from the motor. Moreover, given
the convertibility aspects of the vehicle, the fuel cell should not
reside in a wing because of the possibility of fuel leaks while the
wing panels are being installed. Similarly, the cell could not be
placed in the nose of the vehicle because it would cause large
center-of-gravity shifts in flight and would be vulnerable to
collisions on the ground. Thus, integration of seat mount and fuel
cell frame is a solution that affords equal and high protection to
both pilot and fuel. The vehicle is designed for a single seat to
keep its weight within the 1200 lbs requirement for certification
as a primary class airplane.
The preferred embodiment of the fuel cell 95 is not a conventional
"tank," but rather a bladder similar to racing-car applications and
built to National Hot Rod Association standards. Typically, such
bladder is made of nylon rubber enclosed in an aluminum box and
filled with foam to retard spillage and ignition. These bladders
are designed to withstand a download of about 16,000 lbs and,
therefore, can also serve as a cushion in the event of a very hard
landing. Using a seat from the race car industry weighing
approximately 13 lbs and a stock racing bladder weighing about 15
lbs dry with fittings, it is estimated that the complete seat and
dry fuel tank assembly will weigh approximately 41.5 lbs, including
the frame.
For road operation, the roadable airplane 10 of the invention is
designed to look as a shrouded three-wheel motorcycle with wings
hung on the sides, as shown in FIGS. 1 and 2. FIGS. 15 and 16
illustrate the vehicle 10 in side and top views without wings. The
aircraft-type fuselage 12 covers the steerable front wheel 98 and
rounded fenders 100 encase the rear traction wheels 80 to provide
aerodynamic performance at high speed, both on the road and in the
air. For road use, the wings 14 are either left behind or carried
attached to the body of the vehicle 10, preferably by means of
prongs 101 (see FIGS. 17 and 19) adapted to mate with the same spar
box 40 used to support the wings for flight. When attached to the
body of the vehicle, the tips 102 (back portion) of the wings 40
partially shroud the propeller 66, while the spar stubs 104
protrude forward on the side of the cockpit.
FIGS. 17-19 show three views of the roadable vehicle 10 of the
invention as an airplane. As most evident from the side and plan
views, the vehicle 10 has the same appearance of an airplane,
specifically similar to a Fairchild Republic A-10A Thunderbolt,
commonly known as the Warthog. The wing and tail configurations of
the vehicle 10 are almost the same, as are their relative areas.
The fenders 100 replace the twin turbofans of a Warthog. For
optimal performance, the following key dimensions and surface areas
have been selected for the preferred embodiment of the vehicle 10
described above:
______________________________________ Length 19 ft 2 in Wing span
25 ft Horizontal tail span 7 ft Wing area 81.4 sq ft Horizontal
tail area 14.0 sq ft Propeller diameter 54 in Road clearance 8.5 in
Cabin length 52 in Cabin width (shoulder) 28.75 in Cabin height
(seat to canopy) 34 in Canopy sill height 33 in
______________________________________
The generous cabin width is provided to facilitate the removal of
the power plant from the motor bay, if necessary. The canopy sill
height is measured from the ground under normal compression of the
struts. This design requires the pilot to step over the canopy sill
to enter the airplane, but the sill is low enough to pose no
problem, even when the wings are hung on the sides.
The aerodynamic parameters corresponding to this design are
calculated to be as follows:
______________________________________ Parasite drag area 3 sq ft
Parasite drag coefficient 0.0368 Wing aspect ratio 7.68 Wing span
efficiency 0.9 Pitch stability margin 0.1625 mac
______________________________________
The vehicle's performance depends mostly on engine power, drag
area, and the maximum lift coefficient of the wing. As mentioned,
the Stratus EA81 engine selected for the prototype is expected to
produce about 100 hp at the propeller. The drag area is estimated
as 3 sq ft, and the maximum lift coefficient of the wings with
flaps deflected is assumed to be 2.0, based on similar designs. The
parasite drag area is a very conservatively
assumed value. For example, the parasite drag area of a Lancair 320
is 1.4 sq ft. Thus, the higher value of 3 sq ft is assigned to the
vehicle of the invention because of the wheel openings under the
fuselage, but it is hoped that such openings can be faired to lower
the drag area and correspondingly decrease the drag coefficient as
well.
Based on aerodynamic calculations, the stick-fixed pitch stability
margin is 16.25% of mean aerodynamic chord. The calculations
include estimates of the fuselage pitch moment as well as effects
of the fuselage and wing wakes on the tail. The airplane is
expected to be highly stable but light enough that control force is
unlikely to be a problem. The stability margin is purposefully high
to allow for possible weight increases in the rear of the vehicle,
if necessary, as often happens during final design of specific
components of pusher propeller aircraft.
Another advantageous characteristic of the vehicle is its stability
and control in side winds. The use of three rather than four wheels
is convenient because the vehicle of the invention is considered a
motorcycle for road use, therefore avoiding the most onerous of the
automotive regulations. Moreover, a fourth wheel is not needed for
stability. The center of gravity of the vehicle is low and close to
the rear wheels (about 23 inches above the ground, not far above
the roll center, with a roll factor of about 0.95 that assures the
vehicle will skid before it overturns); therefore, a fourth wheel
would afford little improvement in roll stability. Even with its
three-wheel design, the vehicle's roll factor of 0.95 is comparable
with that of modern sports utility vehicles. In addition, a single
steerable front wheel is easier to cowl than two steerable side
wheels.
Based on the components currently selected and the utilization of a
layered graphite-epoxy/honeycomb-filler skin, about 0.20 inches
thick, for the aeroshell; a 0.028-inch aluminum skin for the bottom
of the craft (to allow radiation to cool the engine); and composite
material, about 0.20 inches thick, for the wing panels; the major
component groups of the vehicle 10, as described, are estimated to
weigh as follows:
______________________________________ Group Weight (lbs)
______________________________________ Useful load 230 Propulsion
404 Rear end 112 Front end 33 Chassis 191 Wings panels 100
Aeroshell 100 Tail 30 Total 1200
______________________________________
The allowed useful load of 230 lbs includes 170 lbs for the pilot
and 60 lbs for 10 gallons of fuel. It is expected that some weight
savings could be realized from surplus weight in the stock Subaru
components, which are designed for much higher weights and torques
than the vehicle of the invention will impose. For example, the
transaxle 48 weighs about 80 lbs, about 70% of the frame weight,
but it is a dual-range unit designed for high torque off road.
Thus, some weight reduction could be obtained by adapting its
structural parameters to the design requirements of the
invention.
Based on these weights and the design characteristics of the
vehicle, the following performance parameters are expected:
______________________________________ Stall speed, flaps down 56
mph Stall speed, flaps up 64 mph Rotation speed 71 mph Takeoff
speed 75 mph Sea level rate of climb 995 fpm Top speed 150 mph
______________________________________
As well understood in the art, the rotation speed is the minimum
indicated air speed at which the horizontal tail can rotate the
airplane under full power at maximum gross weight. The speed is
rather high for the roadable aircraft of the invention because of
the pitch-down moment of the propeller, but it is expected that the
pilot will be able to rotate at a lower speed simply by taking his
foot off the throttle.
Through the dual outputs of the transaxle 48, the rear wheels 80
and propeller 66 are coupled together so that the propeller absorbs
full power at a takeoff speed of about 75 mph. Under propeller
propulsion alone, the thrust at zero speed is estimated to be about
480 lbs, so the initial acceleration would be 480/1200 times
gravity, or 0.4 g. Thrust and acceleration decline as the vehicle
picks up speed, resulting in the acceleration profile 102 shown in
FIG. 20. Thus, the acceleration is only about 0.18 g at 75 mph. The
vehicle is estimated to take about 12.2 seconds and travel 760 ft
to reach takeoff speed under propeller thrust alone.
Supposing the propeller were feathered for zero thrust and the rear
wheels received full power (and the friction coefficient of the
tires were 1.0), the torque available at low speed could provide an
initial acceleration of about 1.0 g. Because the front end bears
only about 180 lbs of weight when the vehicle is stationary,
though, an attempt to accelerate at 1.0 g would lift the front
wheel from the ground which, with some practice, would require a
pilot to be able to control the throttle and clutch to retain an
appreciable load, say 50 lbs, on the front end. The result would
approximate the acceleration profile 104 in FIG. 20. Acceleration
would start at about 0.48 g, but still decrease as the vehicle
picked up speed, partly because the aerodynamic drag offloads the
front end and partly because aerodynamic lift reduces traction.
Thus, powered by wheels alone, the vehicle would attain the takeoff
speed after a time of about 9.7 sec and a distance of 618 ft.
With the wheels and propeller coupled, the vehicle of the invention
will receive thrust from both. Thrust from the wheels is expected
to dominate at low speeds, with an initial acceleration of about
0.48 g when the load on the front end is about 50 lbs. Thrust rises
as the square of speed; thus, as thrust rises, it will push the
front end down and allow the pilot to apply more throttle, which
will results in the approximate profile 106 shown in FIG. 20. The
pilot would have the throttle to the floor at about 48 mph, and
acceleration would decline inversely with speed thereafter. Under
this scenario, the vehicle would reach takeoff speed after a time
of about 8.2 sec and a distance of 510 ft. As a result of its novel
dynamic characteristics, the roadable airplane of the invention is
expected to reach zero to 60 mph in about 5.6 sec.
Given the hybrid design of the vehicle of the invention, most of
the controls and functions needed for flight are also used on the
road. The only exceptions while driving are the wing lift and roll
controls, which are not available when the wings are not installed
in the spar box. Similarly, with the exception of front wheel
steering, all the road functions are also used in flight.
To operate the roadable airplane as a streamlined motorcycle, the
driver raises the canopy and steps over the side into the cabin.
The joystick is moved from side to side to steer the front wheel,
while for and aft motion controls the elevator, and push buttons on
the sides of the stick control the rudders. The actuation of all
these components is achieved through servomechanisms well known in
the art. A gear shift is positioned to the driver's right in
conventional automotive fashion, and pedals for throttle, clutch
and brakes are provided at his feet, just as in an automobile.
In operation, the transaxle's gears are engaged to power either the
wheels only or also the propeller (the latter mode corresponding to
four-wheel drive in automotive terminology); the driver releases a
hand brake on his left, shifts into reverse, backs into the street,
and pauses. The wheels are still, and so is the propeller. The
driver shifts into first gear and accelerates carefully at first,
since the nose is light and can lift under high acceleration. If
connected, the propeller turns at about three times the rate of the
wheels. In such case, the propeller is coupled to the rear wheels
by the belt drive and always turns when they do.
As the vehicle picks up speed, the propeller, if engaged, develops
thrust and pushes the nose down. At about 75 mph and above, the
propeller absorbs all the power of the engine and the front wheel
presses firmly on the ground. The driver can further load the nose
by pushing forward on the stick to exploit the air flow on the
elevator. Since power absorbed by the propeller varies as the cube
of vehicle speed, at 37.5 mph the propeller will absorb only one
eighth of the engine power and the fraction will rise to unity at
about takeoff speed. Because of the configuration of the power
train of the invention, no special means are needed to accomplish
the power transfer from wheels to propeller. The transfer is
entirely aerodynamic.
As well understood in the art, the sound power level of the
propeller varies approximately as the sixth power of vehicle speed.
Thus, the propeller is expected to be silent at idle and
practically silent at normal highway speeds. At the takeoff speed
of about 75 mph, the blade tips have a Mach number of 0.7, well
below the speed of sound, and the sound power is expected to be
modest even then.
To operate the vehicle as an airplane, obviously the wings have to
be installed into the sides of the fuselage and the flap controls
connected to it. The pilot accelerates through first and second
gear, then holds the airplane in third gear until the engine speed
is about 5000 rpm and the airspeed is about 75 mph. He then pulls
back on the joystick to cause the vehicle to start flying. The
airplane climbs in third gear until it reaches cruise altitude. The
pilot then pushes forward on the stick and shifts into fourth gear
as the flight path levels. Thus, through these shifts between
gears, the transaxle of the invention provides the same function as
a variable-pitch propeller, enabling the pilot to maximize engine
power at various propeller speeds. In flight, the rear wheels
rotate with the propeller and serve to buffer the propeller drive
train from engine torsionals. Because the wheels have a much
smaller angular momentum than the propeller, they have no
appreciable gyroscopic effect on pitch and their effect on roll and
yaw are expected to be hardly noticeable. To prepare for landing,
the pilot eases the throttle pedal and may even downshift if he
wants to increase the sink rate. He then depresses the clutch just
before touchdown to allow the propeller and rear wheels to
freewheel together and speed up as necessary when the wheels touch
ground.
While the present invention has been shown and described herein in
what is believed to be the most practical and preferred
embodiments, it is recognized that departures can be made therefrom
within the scope of the invention, which is therefore not to be
limited to the details disclosed herein but is to be accorded the
full scope of the claims so as to embrace any and all equivalent
apparatus and methods.
* * * * *